The present invention relates to coatings comprising a polymer and a mixture of carbon blacks as resistive fillers. The mixture of carbon blacks comprises more than one or at least two different varieties or types of carbon black. Additional fillers can be used in addition to the mixture of carbon blacks. The coatings allow for tailoring of resistivity for use of the coatings in components useful in xerographic, including digital, processes. In preferred embodiments, the coatings are useful as coatings for intermediate transfer components or biasable transfer components, and more specifically, transfer components useful in transferring a developed image in an electrostatographic, especially xerographic machine or apparatus. In embodiments, the present coatings allow for the preparation and manufacture of coated components having excellent electrical, chemical and mechanical properties, including resistivity tailored to a desired resistivity range and excellent conformability. Moreover, intermediate transfer components comprising the mixed carbon black coatings, in embodiments, allow for high transfer efficiencies to and from intermediates even for full color images, and can be useful in both dry and liquid toner development systems.
In a typical electrostatographic reproducing apparatus, a light image of an original to be copied is recorded in the form of an electrostatic latent image upon a photosensitive member and the latent image is subsequently rendered visible by the application of electroscopic thermoplastic resin particles which are commonly referred to as toner. The developed image is then transferred to a copy sheet, or alternatively, is transferred to an intermediate transfer sheet prior to transfer to a copy sheet. The subsequently transferred image is permanently fused to the copy sheet by moving the copy sheet between a heated fusing member in pressure contact with a pressure member.
An important aspect of the transfer process in the electrostatographic process focuses on maintaining the same pattern and intensity of electrostatic fields as on the original latent electrostatic image being reproduced to induce transfer without causing scattering or smearing of the developer material. This important and difficult criterion is satisfied by careful control of the electrostatic fields, which, by necessity, should be high enough to effect toner transfer while being low enough to not cause arcing or excessive ionization at undesired locations. These electrical disturbances can create copy or print defects by inhibiting toner transfer or by inducing uncontrolled transfer which can easily cause scattering or smearing of the development materials. Specifically, excessively high transfer fields can result in premature toner transfer across the air gap, leading to decreased resolution or blurred images. High transfer fields in the pre-nip air gap can also cause ionization, which may lead to loss of transfer efficiency, strobing or other image defects, and a lower latitude of system operating parameters. Conversely, in the post transfer air gap region or the so-called post-nip region at the photoconductor-copy sheet separation area, insufficient transfer fields can give rise to image dropout and may generate hollow characters.
Attempts at controlling the resistivity of intermediate transfer members have been accomplished by, for example, adding conductive fillers such as ionic additives and/or carbon black to the conformable layer.
U.S. Pat. No. 3,959,574 discloses controlling the resistivity of polyurethane coating on a biasable member by use of ionic additives incorporated into the polyurethane. Barium titanate is disclosed as a plasticizer used to control resistivity of the polyurethanes.
U.S. Pat. No. 5,454,980 discloses a method of making an electrically conductive polyurethane elastomer which may be used in a bias transfer member. The polyurethane elastomer may include inorganic pigments such as barium titanate therein.
U.S. Pat. NO. 5,064,509 discloses a process for preparing a multi-layered belt which includes a thermoplastic film forming polymer which may be comprised of polyurethane or prepolymers of polyimide and which may include conductive particles such as carbon black, graphite or titanium dispersed therein.
Generally, carbon additives tend to control the resistivities and provide somewhat stable resistivities upon changes in temperature, relative humidity, running time, and leaching out of contamination to photoconductors. However, the required tolerance in the filler loading to achieve the required range of resistivity has been extremely narrow. In other words, a small change in percentage of carbon black filler loading has lead to a large change in resistivity. This, along with the large "batch to batch" variation, leads to the need for extremely tight resistivity control. In addition, carbon filled polymer surfaces have typically had very poor dielectric strength and sometimes significant resistivity dependence on applied fields. This leads to a compromise in the choice of centerline resistivity due to the variability in the electrical properties, which in turn, ultimately leads to a compromise in performance.
Therefore, there exists an overall need for a coating which can be tailored to a specific resistivity and/or dielectric strength, and wherein a relatively small change in filler loading will not significantly affect the resistivity and/or dielectric strength.
The present invention, in embodiments, allows for tailoring of specific and desired resistivities in order to increase transfer efficiency and to decrease the above discussed problems in inefficient transfer. The present invention, in embodiments, solves the above problems by providing transfer members, including bias transfer members and intermediate transfer members, which comprise a polymer and a mixture of carbon blacks dispersed therein. The combination of polymer and mixture of different carbon blacks allows for sufficient tailoring of desired resistivities.